Never done this myself but
AFAIK you just get the IMCR application form (the same form which you use to apply for the rating following a successful checkride) from the CAA, and there is a section which asks whether you are applying for the IMCR using a IR as a basis or whatever; you put Yes in there, and a cheque for £70 or so, and that's it.
As I said, you need a valid UK/JAA license and a valid UK medical.
Incidentally, the IMCR is equally valid in an N-reg; I have this in writing from both the CAA (who correctly say it's up to the FAA) and the FAA.